Well, last year, the tournaments were moved up by about a week (or, rather, 2/3 of a week). So if they dropped off the points from Rome a week earlier than Rome was played this year, this would result in Nadal losing a lot of points last week and then gaining them back this week.

Federer had, last year, won Hamburg, which started on May 9th; this year, Rome started on May 8th, so maybe if Hamburg got dropped the same week that New Rome got added, then it would result in a slight net drop for Roger.

I'm not sure exactly - I would guess it's something like that, to do with tournaments not being played at the exact same time each year. Those aren't official accounts, just me looking at the dates and making things up